1. Field of the Invention
This invention relates to an oil-filled capacitor which is impregnated with an electrical insulating oil that is prepared from the by-product oil fractions obtained in the process to prepare ethylbenzene. Furthermore, the invention relates to an oil-filled capacitor which is impregnated with an electrical insulating oil comprising a specific fraction or its mixture derived from the above-mentioned by-product oil.
2. Description of the Prior Art
It is well known that a heavy by-product oil is produced in the process to prepare ethylbenzene by reacting benzene with ethylene in the presence of an alkylation catalyst as disclosed in, for example, U.S. Pat. Nos. 4,108,788; 4,111,825; and 4,228,024.
In the same reference, it is described that the fraction having a boiling range of 275 to 420.degree. C. is useful as an electrical insulating oil.
The present inventors recovered the fraction described in the above reference by distillation and used it for the impregnation of oil-filled capacitors made by using polypropylene film. However, it was found out that capacitors having excellent characteristics cannot always be made, while the reason for this has not yet been clear.
The above by-product oil obtained from the ethylbenzene preparation process, however, inevitably contains various uncertain compounds that cannot be determined by analysis, as the general characters of the materials of this kind. The kinds and quantities of these undeterminable compounds are not fixed and, in addition, the boiling points of these compounds are close to or overlapped with one another. Accordingly, it is impossible to isolate any component only by distillation from the by-product oil.
In the case that a fraction having a certain boiling point is recovered, the boiling point of the fraction is indicated by the distilling temperature of the fraction. In practice, however, the efficiency of the distillation generally depends upon the distilling conditions such as the number of theoretical plates of distillation apparatus, reflux ratio, bottom temperature and other temperature distribution in a distillation column, and the rate of distillation. Even when the distillation temperatures, that is, the boiling points are the same, the kinds and quantities of components in distilled fractions are liable to vary largely.
Owing to the complexity of the origin of by-product oil and operational factors in the distillation of by-product oil, it is supposed that an electrical insulating oil having excellent characteristics cannot be obtained only by a simple measure of distillation.